01 - Preheat oven to 350°F. Grease and flour a 10- to 12-cup bundt pan thoroughly.
02 - In a medium bowl, whisk flour, baking powder, baking soda, salt, and poppy seeds; set aside.
03 - Using an electric mixer on medium speed, beat butter and sugar until light and fluffy, approximately 3 to 4 minutes.
04 - Add eggs one at a time, beating well after each addition. Mix in lemon zest and vanilla extract.
05 - In a small bowl, stir together buttermilk and lemon juice.
06 - With mixer on low, add flour mixture and buttermilk mixture alternately in three parts, starting and ending with flour mixture; mix until just combined without overmixing.
07 - Pour batter evenly into prepared pan, smoothing the top.
08 - Bake for 40 to 45 minutes, until a toothpick inserted in the center emerges clean.
09 - Allow cake to cool in pan for 15 minutes, then invert onto a cooling rack to cool completely.
10 - Whisk powdered sugar, lemon juice, and lemon zest until smooth and pourable; adjust lemon juice to reach desired consistency.
11 - Drizzle glaze evenly over the cooled cake and let it set before slicing and serving.